1. Field of the Invention
The present invention relates to short range wireless communication systems. More particularly, the present invention relates to short range inductively coupled communication systems.
2. Description of the Prior Art and Related Information
Short range wireless communication systems are of significant importance for a variety of applications. Although many applications are quite common, such as cordless telephones, "walkie talkies" and cordless microphones, a large number of other potential applications also exist. Such potential applications, which could include virtually any system employing audio cord coupling, have generally not been exploited due to limitations in existing short range communication systems.
There are four basic wireless communication systems in use today: acoustic, radio frequency, infrared and inductive. Each of these has performance characteristics, and attendant advantages and limitations, well known to those skilled in the art. For example, acoustic communication is short range, can cover large groups of people and is subject to interference. Radio frequency (or RF) communication in turn is long range and can use many forms of modulation including amplitude modulation (AM) and frequency modulation (FM) for transmitting signals on an RF carrier. Infrared communication is generally limited to short range, line of sight communication and is immune to forms of interference other than light. The cost vs. range vs. fidelity of each method is well known to those skilled in the art.
The last noted approach to short range wireless communication employs inductively coupled, or magnetically coupled, wireless transmission. In particular, this approach has been employed in hearing assistive devices for communication between a belt worn radio and a small hearing aid of the type worn behind the ear (BTE) or in the ear (ITE). Cordless coupling between the belt worn device and the ITE or BTE hearing aid is important both for aesthetic reasons, in view of the fact that hearing aid wearers usually desire to avoid calling attention to their hearing disability, as well as for practical reasons relating to awkwardness relative to a cord directly attached to a BTE or ITE hearing aid.
In such a hearing assistive application, the belt worn receiver or microphone is coupled via a cord or cable to a loop which is worn under the clothing of the hearing aid user and encircles the user's neck. The ITE or BTE hearing aid in turn has an inductive pick up loop therein, for example, the inductive loop employed for picking up telephone audio (so called teleloop, T-loop or T-coil), to pick up the audio signal from the neck worn transmission loop. The audio signals from the belt worn audio source directly amplitude modulate the current through the neck worn loop to produce an amplitude modulated signal in the audio frequency band. This is commonly referred to as a base band amplitude modulation system, since no carrier frequency is employed (as in the case of RF transmission). The use of an inductive loop allows connecting cords to be omitted from the ITE or BTE hearing aid thus providing the advantages of aesthetics, i.e., removing visible cords from the hearing impaired user, as well as convenience due to the lack of cords which may become tangled or interfere with the actions of the user.
One advantage of an inductively coupled system relative to an RF transmission system is that since that such an inductively coupled system operates in the audio frequency range, FCC regulation is not applicable and problems related to FCC licensing are avoided. Another advantage of inductive coupling for short-range communication systems of this type is that low power consumption is associated with the system. Due to the desired small size of the hearing aid, low power consumption is a key requirement since any batteries employed must be of small size and hence limited in power supply capabilities. Also, the transmitter portion of the system, in the noted example a belt worn device, may be battery operated and hence sensitive to power consumption.
A significant disadvantage of an inductively coupled base band amplitude modulated system is the tendency to experience signal variations due to changes in the relative orientation of the receiving coil and the transmission loop as well as sensitivity to distance between the receiving coil and transmission loop. This not only causes fading in and out of the signal as the user moves about, but also creates serious signal to noise problems where lower magnitude audio signals are being transmitted.
In addition, the electrical inductance of either the transmitting or receiving coil is inversely related to the bandwidth of the system. Increasing the electrical inductance reduces the bandwidth, providing a "Catch-22" where wide bandwidth low power transmission is desired.
Another problem of base band inductively coupled systems is the high background noise typically present due to the magnetic fields of common electrical devices. A base band magnetic element (pick up coil) typically has its resonance in the audio range, and a typical system has a sensitivity around 2 milliGauss (mG). Electrical appliances, motors, fluorescent lights produce a magnetic field in the 60-120 Hz range, which overlaps the audio range and can be detected in base band operation. The following are typical magnetic field strength of a variety of electrical devices, measured at a distance of 3 feet.
______________________________________ Fluorescent Lights 3 mG Electric Toaster 3 mG Electric Motor 2 mG House Wiring 2 mG Electric Blanket 100 mG ______________________________________
When compared to an inductive coupled base band system sensitivity of about 2 mG, it is clear these items have enough of a magnetic electric field to cause significant interference in base band systems.
There are thus several serious limitations associated with base band systems: signal fading with varying distance and orientation of the receiver with respect to the loop; signal to noise ratio deterioration with distance; restrictions on the signal bandwidth by parameters of the circuit elements; and presence of high background noise due to common electrical devices. Therefore, the quality of audio communication provided by base band systems has been less than desired.
Accordingly, it will be appreciated that a need presently exists for a short range wireless communication system which consumes relatively little power and which can be implemented without complex circuitry. It will be further appreciated that a need presently exists for a short range wireless communication system which provides good audio quality reception and which is not subject to the above-noted problems. It will be further appreciated that a need presently exists for a short range wireless communication system which is not subject to federal regulations involving long range communication systems.